Computing scene luminance
I think what you want is &quot;linear light&quot; and a linear colour model.
Setting either &quot;Displace Device&quot; to &quot;None&quot; or &quot;View Transform&quot; to &quot;Raw&quot; under &quot;Color Management&quot; in &quot;Render Properties&quot; seems to do the trick:
 
Also pay attention to &quot;Save As Render&quot; in the image save dialog box.
A render with four concentric and equally spaced rings with maximum roughness and zero specularity around a point light source produces peak colour values of around [1.0, 0.29, 0.14, 0.09]— Mostly what you'd expect from physically correct inverse-square behaviour, with some margin from clipping and sampling noise:

I saved the result as a 16-bit PNG, which should give you plenty of bit depth to play with. You may also want to try EXR or RGBE, depending on your use case— My image viewer for those formats seemed to apply an sRGB transform by default on display, but the data itself seems to be intact.




Notes on interpreting the data:
As the output data is in the form of dimensionless RGB values, if you want physical units then you'll have to calculate a conversion ratio based off of a known lamp power output, surface albedo, and distance.
In my example file, the innermost ring has an RGB peak of 1.0 with an albedo of 0.8 at a distance of 1m from a point light source with 50W of power output.
The 50W of radiation at a distance of 1m will be spread across a spherical shell of 4π(1m)²=12.57m², so incoming irradiance at that distance should be 50W/(12.57m²)=3.98W/m². Outgoing surface radiosity accounting for the 0.8 albedo should then be 3.98W/m²*0.8=3.18W/m². So that means RGB(1.0)=3.18W/m², which in turn means that the radiosity at the other rings can be figured out by RGB(0.29)=(0.29/1.0)*3.18W/m²=0.92W/m², etc.
It may be more sensible to establish a conversion ratio using a Sun lamp shining perpendicularly on a full-albedo, full-roughness, zero-fresnel dielectric (non-metallic) surface than a point lamp. The tooltip says that &quot;Strength&quot; maps directly to W/m².
You can probably also put that ratios or its multiple and/or reciprocal into the &quot;Exposure&quot; setting under Colour Management in order to havea 1:1 ratio between the RGB values and your physical unit of choice. Note that if RGB goes above 1.0, then you'll probably want to use either EXR or RGBE as your output file format.
Always sample parts of your data by hand and check your numbers manually to see whether they make any sense, both based on your scene and relative to each other. Digital colour management is frankly a mess, and there's a lot that can go wrong at pretty much any stage of the pipeline when you're trying to pull physical measurements out of artistic software.
Further caveats:
Note that this also still technically isn't the luminance, as it's just the radiosity. Luminance is a measure of perceived light as detected by human retinal cells, so you'd need to apply a luminosity function to calculate it. You'll also need to make sure your render engine, file format, image viewer, etc aren't already accounting for the effects of the luminosity function.
From the fact that RGB(1,1,1) is white, we can also reason that even if interpreted as radiosity, the data is scaled against sunlight's spectrum rather than having a spectrally uniform power distribution.
Also note that Cycles and EEVEE run entirely in RGB, AFAIK, and not with continuous spectra. So if the spectra of your light sources and albedoes have a lot of spikes or valleys, the result will end up being further from physical accuracy.